If :only or :except are used, convert the options into the primitive form
(:per_key) used by ActiveSupport::Callbacks. The
basic idea is that :only => :index gets converted to :if => proc {|c|
c.action_name == “index” }, but that the proc is only evaluated once
per action for the lifetime of a Rails process.

Options

only - The callback should be run only for this action

except - The callback should be run for all actions except this
action